Synthesis, Purification, and Rotational Spectroscopy of (Cyanomethylene)CyclopropaneAn Isomer of Pyridine

The
gas-phase rotational spectrum of (cyanomethylene)­cyclopropane,
(CH2)2CCHCN, generated by a Wittig reaction
between the hemiketal of cyclopropanone and (cyanomethylene)­triphenylphosphorane,
is presented for the first time. This small, highly polar nitrile
is a cyclopropyl-containing structural isomer of pyridine. The rotational
spectra of the ground state and two vibrationally excited states were
observed, analyzed, and least-squares fit from 130 to 360 GHz. Over
3900 R-, P-, and Q-branch, ground-state rotational transitions were
fit to low-error, partial octic, A- and S-reduced Hamiltonians, providing
precise determinations of the spectroscopic constants. The two lowest-energy
vibrationally excited states, ν17 and ν27, form a Coriolis-coupled dyad displaying small a- and b-type resonances. Transitions for these two
states were measured and least-squares fit to a two-state, partial
octic, A-reduced Hamiltonian in the Ir representation with
nine Coriolis-coupling terms (Ga, GaJ, GaK, GaJJ, Fbc, FbcJ, FbcK, Gb, and GbJ). The observation of many resonant transitions
and nine nominal interstate transitions enabled a very accurate and
precise energy difference between ν17 and ν27 to be determined: ΔE17,27 = 29.8975453 (33) cm–1. The spectroscopic constants
presented herein provide the foundation for future astronomical searches
for (cyanomethylene)­cyclopropane.